My guess is that the FBI originally had access, changed the I.D. and password and lost them. Then all the nonsense to force Apple to help. And now, after going through tons of paper waste from the office, the FBI found the scrap of paper where they scribbled the I.D. and password. Bunch of keystone cops.
I’m thinking none of them wanted to sink low enough to call the NSA for help, so they finally cracked the phone after months of effort. Meanwhile, the USG has experts that could’ve cracked it over their lunch break.